The Bcl-2 family constitutes a crucial checkpoint in the cell death pathway. BID is a member of the "BH3- only" subset of this family and requires its conserved BH3 domain for its pro-apoptotic function. BID plays an important role in the apoptotic pathway downstream of death receptors such as TNF R1 and Fas. Cleavage by caspase 8 and myristoylation serve to activate BID, facilitating targeting to the mitochondria where it activates multidomain pro-apoptotic family members such as BAX or BAK, and activating the downstream apoptotMice in which Bid has been deleted undergo all normal developmental deaths. Aging Bid- deficient mice fail to maintain myeloid homeostasis, progressing to a fatal, clonal disorder that closely resembles human Chronic Myelomonocytic Leukemia (CMML). Thus, this single "BH3-only" protein thus plays a critical role in maintenance of normal myeloid homeostasis and tumor suppression. In addition to its role in apoptosis, we have uncovered a role for BID in regulating the DNA damage-induced intra-S phase checkpoint that does not require its death-promoting BH3 domain. Following DNA damage, BID is found in the nucleus, phosphorylated by ATM, and plays a role in the intra-S phase checkpoint. Thus, BID has two distinct and separable functions, an apoptotic function mediated by caspase cleavage and its BHS-domain, and a cell cycle/DMA repair function mediated by phosphorylation by the DNA damage kinase ATM. One of the central problems in the DNA damage and apoptotic pathways is how cells determine their response to DNA damage. Some cells arrest the cell cycle, and others undergo apoptosis. We hypothesize that BID acts at the interface between the DNA damage response and apoptosis, in position to execute the decision of a cell to undergo cell cycle arrest and initiate DNA repair or to undergo apoptosis. We propose to directly test this hypothesis by dissecting the mechanism of Bid function on both sides of this molecular switch. An understanding of the regulatory mechanisms that govern a cell's decision to activate cell cycle checkpoints or to undergo apoptosis has important implications for how cells respond to current cancer therapy, and should lead to important clues to new therapeutic targets.